The invention relates generally to marine seismic prospecting and more particularly to shipboard cable-handling systems for retrieving instrumented marine cables.
Marine cables instrumented with hydrophones and other sensors are used in prospecting for oil and gas under the sea floor. Streamers are instrumented cables towed behind a survey vessel to detect reflections of periodic seismic blasts off geologic structures under the sea floor. Ocean bottom cables (OBCs) are instrumented cables laid on the sea floor by a deployment vessel to detect reflections of seismic blasts. After deployment, streamers and OBCs have to be retrieved by the vessel and stored.
Conventional cable-handling systems apply tension to the cables by frictionally pulling them aboard the vessel. In the case of streamers, the cable-handling systems frictionally engage the outer skins, or jackets, of the streamers, causing internal shear forces that can damage the sensors, wires, and other streamer components. In the case of OBCs, the cable-handling systems frictionally engage the outer armor layer or an outer jacket, damaging and wearing these outer components and shortening the lifetimes of the cables. Streamers and OBCs both have rigid sections that, while under the tension caused by cable-handling systems, have to pass over curved guides or surfaces on the vessel during retrieval. The cable-handling systems thus create high bending loads in the rigid sections as they pass over curved surfaces under tension. If great enough, the bending loads can damage the rigid sections.
Thus, there is a need to prevent damage to instrumented marine cables during retrieval by a deployment vessel.